Leptin is produced solely in adipose tissue and appears to serve as a regulator of several metabolic, hypothalamic and anterior pituitary functions. In humans, leptin levels rise in concordance with adiposity as measured by body mass index and levels correlate most closely with subcutaneous fat stores. We will compare baseline leptin levels and body composition in obese children to those available in nonobese peers and study the effects of metformin on these parameters in obese children with hyperinsulinism.